prepare(self)

Called just before the task is executed.
This is mainly intended to give the target Nodes a chance to
unlink underlying files and make all necessary directories before
the Action is actually called to build the targets.

executed(self)

Called when the task has been successfully executed and
the Taskmaster instance wants to call the Node's callback
methods.
This may have been a do-nothing operation (to preserve build
order), so we must check the node's state before deciding whether
it was "built", in which case we call the appropriate Node method.
In any event, we always call "visited()", which will handle any
post-visit actions that must take place regardless of whether
or not the target was an actual built target or a source Node.